We have performed long term studies on mechanical pulmonary ventilation in laboratory animals, exploring the effects of 50 and 30 H20 peak airway pressure. At a peak airway pressure of 50 cm H20 there was uniformly a period of substantial improvement in lung function and lung mechanics over the first few hours of mechanical pulmonary ventilation; following this initial improvement there was progressive deterioration in all measured variables, leading to severe acute respiratory failure. In similar studies at peak airway pressure of 30 cm H20, there was a lesser though similar course, leading to respiratory failure in all animals so ventilated. We wish to implicate the mechanical pulmonary ventilator at pressures at least as low as 30 cm H20 and its use-abuse due to lack of previously available information as the major factor responsible for the delay, or the lack of healing, in such diverse disease states (all treated while on the ventilator) as adult respiratory distress syndrome, hyaline membrane disease, bronchopulmonary dysplasia. The judicious use of the "third lung" (membrane artificial lung) can offer a means to avoid resorting to mechanical pulmonary ventilation permitting diseased lungs to fully recover.